The site of the Civil War battle at Prairie Grove is now a state park.

Washington County began as part of the Cherokee Territory, following an 1817 treaty. The area was next known as Lovely County, and one year later Washington County was created after another Cherokee treaty. The court house was centrally located in the city of Washington, modern-day Fayetteville (renamed to avoid confusion with Washington, Arkansas in South Arkansas). The Lee Creek Valley in southern Washington County contained many of the county's early settlements, including Cane Hill and Evansville.[3]

The economy of Washington County was based on apples in the late 19th century. A mixture of wet weather, altitude, and loamy soils provided a good environment for apple orchards.[5] First planted in areas around Lincoln, Evansville, and Cane Hill in the 1830s, apple orchards began all across the county. The United States Census reported a crop of 614,924 bushels of apples produced by the county in 1900, the highest in the state. Several varieties of apple were discovered in the area including Shannon Pippin, Wilson June, and most notably the Arkansas Black.[6] The Ben Davis became the apple of choice in the area for sale and shipment across the region. Corn became the dominant crop, outselling apples by almost $500,000 in 1900.[7]

Arkansas Industrial University was founded in the growing community of Fayetteville in 1871 after William McIlroy a donated farmland for the site. The university changed its name in 1899 to the University of Arkansas.[8] Railroads came to Washington County after the St. Louis – San Francisco Railway (Frisco) decided to build a line to Texas through Fort Smith. Two possible routes were proposed, one passing through Prairie Grove, the other through Fayetteville. Many Fayetteville residents and farmers sold or donated land for the right of way to influence the choice. They were successful and in 1881 the first passenger train arrived at Fayetteville.[9] The county continued to grow with more churches and schools after the railroad's completion.[10] Rural parts of the county began losing population in the 1920s during the Great Depression, when high taxes forcing residents to move to Fayetteville or west to Oklahoma. The rural areas later became the Ozark National Forest and Devil's Den State Park.[10]

Washington County sits on a basement of Precambriangranite and rhyolite, as most of the continental interior of the United States does.[13] Much of the county's geologic history must be inferred from nearby Oklahoma and Missouri research, due to the steepness of the more recently formed mountains that did not form in the neighboring states. This igneous material was eroded until the Paleozoic, when oceans covered the now-low lying area.[14] These oceans came and retreated for 300 million years, depositing various different sedements during that time. This created fossiliferouslimestone and ripple marked-sandstone, both present throughout the north part of the county as evidence of ancient oceans.[14]

Sediments were deposited from the Devonian, Mississippian, and Pennsylvanian periods. During this deposition period, the county had a climate similar to that of the present-day Bahamas, as the equator was north of Washington County.[15] The Devonian brought mostly shales, the Mississippian brought the limestones and chert visible in the bluffs. This chert is present throughout most of the county. The county is also home to the Boone Formation (red soils), white limestones, the Wedington Sandstone, the Bastesville Sandstone, the Pitkin formation (ocean-fossil limestone), and the Fayetteville Shale.

Settlers were attracted to the area by its numerous streams, used to power gristmills, sandstones and clays for use in construction, lime-sweetened soil, and chert for road construction.[16]

Today, Washington County consists of two main formations, the Boston Mountains and the Springfield Plateau. During the late Pennsylvanian, sediments were deposited on top of the Springfield Plateau. The area was uplifted during the Ouachita orogeny and subsequent erosion formed the rugged Boston Mountains. Erosion of these sediments causes the Boston Mountains to be carved steeply in the south, while in the north of the county, the Boston Mountain sediments are almost entirely eroded, exposing the older rocks of the Springfield Plateau.

There were 60,151 households out of which 32.50% had children under the age of 18 living with them, 52.30% were married couples living together, 9.40% had a female householder with no husband present, and 34.40% were non-families. 25.80% of all households were made up of individuals and 7.10% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 3.07.

In the county the population was spread out with 25.00% under the age of 18, 15.30% from 18 to 24, 30.20% from 25 to 44, 19.50% from 45 to 64, and 9.90% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 100.40 males. For every 100 females age 18 and over, there were 98.70 males.

The median income for a household in the county was $34,691, and the median income for a family was $42,795. Males had a median income of $29,428 versus $21,769 for females. The per capita income for the county was $17,347. About 9.40% of families and 14.60% of the population were below the poverty line, including 16.50% of those under age 18 and 10.20% of those age 65 or over.

As of the 2010 Census Washington County had a population of 203,065. The racial makeup of the county was 74.14% Non-Hispanic white, 2.96% black, 1.18% Native American, 2.17% Asian, 2.03% Pacific Islander, 0.11% non-Hispanics of some other race, 2.16% Non-Hispanics of two or more races and 15.49% Hispanic.

The University of Arkansas at Fayetteville was founded in 1871 on the site of a hilltop farm that overlooked the Ozark Mountains, giving it the nickname "The Hill".[33] The University of Arkansas is in Fayetteville, in Washington County. Historically, Cane Hill College in Canehill was the first college in Arkansas, prior to the University of Arkansas's founding in 1871. Canehill probably influenced the placing the University of Arkansas Washington County, since the history of education in the county was a major factor in the decision.

Townships in Arkansas are the divisions of a county. Each township includes unincorporated areas and some may have incorporated towns or cities within part of their space. Townships have limited purposes in modern times. However, they are of value for historical purposes in terms of genealogical research. Each town or city is within one or more townships in an Arkansas county based on census maps. The townships of Washington County are listed below. In Washington County, each incorporated town/city is at least partially located within its namesake township.[34][35]